pkg_resources.ExtractionError: Can't extract file(s) to egg cache
The following error occurred while trying to extract file(s) to the Python egg
cache:
[Errno 17] File exists: '/home/bamboo/.python-eggs'
The Python egg cache directory is currently set to:
/home/bamboo/.python-eggs
Perhaps your account does not have write access to this directory? You can
change the cache directory by setting the PYTHON_EGG_CACHE environment
variable to point to an accessible directory.

After a little research, I decided that instead of installing PyCrypto
as a zipped egg (as it does by default) into the build’s virtual en­vi­ron­ment,
to instead force it to unzip itself: easy_in­stall --always-unzip pycrypto.
This seems to have fixed the problem,
as pkg_re­sources no longer needs to unpack anything.
Pillow was also in the Egg Cache and probably needs similar treatment.